Dual Tone Multi-Frequency (DTMF) detectors have become widely used in the telecommunication industry. DTMF signals include two tones, one from a row group of frequencies and one from a column group of other frequencies. A pair of frequencies (one from the row and one from the column) determine a symbol. In one illustrative example, four frequencies may be selected for the row group and four frequencies may be selected for the column group. Sixteen pairs can be created from this grouping and can represent sixteen symbols, for instance, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, a, b, c, d, *, and #. The row frequencies may be 697 Hz, 770 Hz, 852 Hz, and 941 Hz. The column frequencies may be 1209 Hz, 1336 Hz, 1477 Hz, and 1633 Hz.
Devices in modern telecommunication systems may use several different methods to detect DTMF tones. For example, one technique uses discrete Fourier transforms (DTFs) to detect DTMF tones. Specifically, the DTF values at only the tone frequencies are computed as specified in a modified Goertzel algorithm. Although this technique may detect the DTMF tones, it has certain limitations. For instance, the technique depends upon the use of finely tuned thresholds and, therefore, may have variations depending upon different operating environments.